As the technique for estimating the friction coefficient (hereinafter referred to simply as “μ” in some cases) of a road surface on which a vehicle is traveling, the techniques disclosed in, for example, U.S. Pat. No. 3,669,668 (hereinafter referred to as “patent document 1”) and Japanese Patent Application Laid-Open No. 2003-118554 (hereinafter referred to as “patent document 2”), have been proposed by the present applicant.
According to the technique disclosed in patent document 1, a road surface reaction force acting on each wheel from a road surface (a cornering force (a lateral force of a vehicle) and a braking/driving force (a longitudinal force of the vehicle)) is estimated using a tire characteristic set on the basis of an estimated value of μ. Then, based on the estimated value of the road surface reaction force, the estimated value of a lateral acceleration of the vehicle and the estimated value of a yaw rate changing velocity of the vehicle (the yaw rate changing velocity at the center of gravity of the vehicle), which are motion state quantities of the vehicle generated by the resultant force of the road surface reaction forces, are calculated.
Further, according to the technique disclosed in patent document 1, a previous estimated value of μ is updated on the basis of the deviation between the value of the lateral acceleration detected by an acceleration sensor and the estimated value of the lateral acceleration or the deviation between the differential value of the yaw rate values detected by the yaw rate sensor (the detected value of the yaw rate changing velocity) and the estimated value of the yaw rate changing velocity, whichever deviation is greater, thereby finding a new estimated value of μ.
According to the technique disclosed in patent document 2, a tire model set on the basis of the estimated value of μ is used to estimate the road surface reaction force acting on each wheel from a road surface (the cornering force and the braking/driving force). Then, based on the estimated value of the road surface reaction force, the estimated value of a lateral acceleration of the vehicle and the estimated value of the longitudinal acceleration of the vehicle, which are the motion state quantities of the vehicle generated by the resultant force of the road surface reaction forces, are calculated.
According to the technique disclosed in patent document 2, in the case where a slip angle (skid angle) of a rear wheel is small, the estimated value of μ is incremented or decremented by a predetermined value according to a magnitude relationship between the estimated value of the longitudinal acceleration of the vehicle and the detected value of the longitudinal acceleration provided by the sensor. In the case where the slip angle of a rear wheel is large, the estimated value of μ is incremented or decremented by a predetermined value according to the magnitude relationship between the estimated value of the lateral acceleration of the vehicle and the detected value of the lateral acceleration provided by the sensor. Thus, the estimated value of μ is sequentially updated.
The road surface reaction force acting on a wheel depends not only on μ but also on the slip rate or the skid angle (slip angle) of a wheel. For this reason, according to the techniques disclosed in patent documents 1 and 2, the slip rate of a wheel is estimated and the skid angle of a vehicle or the skid angle of a wheel is also estimated using a motion model of the vehicle.
The techniques disclosed in patent documents 1 and 2 are based on the assumption that the deviation between the estimated value of the lateral acceleration of the center of gravity of the vehicle and the value of lateral acceleration detected by the acceleration sensor (hereinafter referred to as the lateral acceleration deviation), the deviation between the estimated value of the longitudinal acceleration of the center of gravity of the vehicle and the value of the longitudinal acceleration detected by the acceleration sensor (hereinafter referred to as the longitudinal acceleration deviation), or the deviation between the estimated value of the yaw rate changing velocity at the center of gravity of the vehicle and the detected value of the yaw rate changing velocity based on an output of a yaw rate sensor (hereinafter referred to as the yaw rate changing velocity deviation) are caused by the error of an estimated value of μ used to find the estimated value of the lateral acceleration, the estimated value of the longitudinal acceleration, or the estimated value of the yaw rate changing velocity.
On the other hand, the detected value of the lateral acceleration, the longitudinal acceleration, and the yaw rate changing velocity often includes a steady offset component caused by a drift of an output of sensors corresponding to them, respectively, or an inclination or the like of a road surface.
This situation sometimes causes a disadvantage as described below in the case of updating the estimated value of μ according to the aforesaid lateral acceleration deviation, the longitudinal acceleration deviation, or the yaw rate changing velocity deviation.
In other words, for example, in the case where the detected value of the lateral acceleration of the vehicle includes an offset component as described above when updating the estimated value of μ according to the lateral acceleration deviation, if the actual lateral acceleration changes in polarity from one of the positive and negative polarities (direction) to the other polarity (direction), a difference occurs between the change timing of the actual lateral acceleration and the change timing of the detected value of the lateral acceleration (so-called zero-cross timing).
Therefore, particularly, around the zero-cross timing of the actual lateral acceleration (in a situation where the actual lateral acceleration reaches zero or a value close to zero), the polarity of the lateral acceleration deviation is often reverse to the actual polarity (the polarity of the deviation between the actual lateral acceleration and the estimated value of the lateral acceleration).
Then, in such a situation, a disadvantage easily occurs that the estimated value of μ decreases though the estimated value of μ should be increased in order to bring the estimated value of μ close to the actual μ value. In other cases, a disadvantage easily occurs that the estimated value of μ increases though the estimated value of μ should be decreased. In other words, a disadvantage easily occurs that the estimated value of μ changes in a direction of being farther away from the actual value of μ.
In addition, the disadvantage occurs in the same manner also in the case of updating the estimated value of μ according to the longitudinal acceleration deviation or the yaw rate changing velocity deviation.
Therefore, the conventional techniques disclosed in patent documents 1 and 2 have had a disadvantage of easily causing a situation where the estimated value of μ diverges from the actual value of μ so as to be farther away from the actual value of μ.